In recent years, a white LED has been widely used, and particularly, a white LED having a blue LED and a yellow phosphor combined is used for a light source for a liquid crystal display of a cell-phone or for an auxiliary light for a camera. A typical white LED has a structure wherein a blue LED is sealed by a mixture of a phosphor powder and a resin (Patent Document 1).
With such white LED, a part of blue light emitted from the blue LED will impinge on and be absorbed by the phosphor which will emit a yellow fluorescence, whereupon white light will be emitted by color mixing of blue and yellow colors which are in a complementary chromaticity relationship. However, this white light does not substantially contain a red color component and lacks also in a green color, and thus it is poor in color reproducibility and has a drawback such that even when it is applied as illuminating light to a red-colored object, it can not be observed as a fine red color. Therefore, such white light is sometimes called pseudo white.
On the other hand, it has been applied to prepare a white LED having color reproducibility improved by mixing three or four colors, for example, by combining a blue LED with a green phosphor and a red phosphor, or by combining an ultraviolet LED with phosphors having light's three primary colors of blue, green and red or further mixing a yellow phosphor thereto. However, there have been various problems, and those industrially produced are mostly pseudo white LED by a combination of a blue LED and a yellow phosphor.
If it is possible to use for illumination a white LED having color reproducibility improved, it is possible not to use mercury which is unavoidably used in a fluorescent lamp, and a long operating life and energy saving can be attained, such being good for the environment. However, at the present stage, the luminous efficiency is far below the level of a fluorescent lamp, and such can not be regarded as contributing to energy saving. Further, if LEDs with current output are to be arranged, many LEDs will be required such being disadvantageous also from the viewpoint of costs as compared with a fluorescent lamp. It is therefore an industrially important object to solve these problems.
In order to improve the luminous efficiency, it is most important to increase the luminous efficiency of blue and ultraviolet LEDs. Further, there are other problems to be solved such that a phosphor having sufficiently high efficiency for this application has not yet been available, the sealing resin is deteriorated by ultraviolet rays whereby the operating life of the light source tends to be shortened, and the light quantity per LED chip is small.
Further, as a phosphor, one employing an oxide, silicate, phosphate, aluminate or sulfide, as the matrix material and employing a transition metal or rare earth element, as the luminescent center, is widely known.
Further, with respect to a white LED, attention has been drawn to phosphors which are excited by an excitation source such as ultraviolet or blue light to emit visible light, and development thereof is under way. However, the above-mentioned conventional phosphors have a problem such that the luminance thereof decreases as a result of being exposed to the excitation source or a high temperature high humidity environment.
Therefore, as phosphors which are less susceptible to deterioration of the luminance, attention has recently been drawn to nitride and oxynitride phosphors, as they are materials having a stable crystal structure and being capable of shifting the excitation light and emission light to the longer wavelength side.
Among the nitride and oxynitride phosphors, an α-sialon activated by a specific rare earth element is known to have useful fluorescence properties, and its application to a white LED or the like is being studied (Patent Documents 2 to 6, and Non-Patent Document 1).
The α-sialon is a solid solution of α-silicon nitride and has a structure wherein specific elements (Ca and Li, Mg, Y or a lanthanide metal except for La and Ce) penetrate into a crystal lattice to form a solid solution, and Si—N bonds are partially substituted by Al—N bonds and Al—O bonds in order to maintain electrical neutrality. When a part of elements penetrating to form a solid solution is a rare earth element which becomes the luminescent center, the fluorescence property is exhibited.
Usually, the α-sialon is obtained by firing a powder mixture comprising silicon-nitride, aluminum-nitride, if necessary aluminum oxide, and oxides of elements penetrating to form a solid solution, at a high temperature in nitrogen. A variety of fluorescence properties can be obtained depending on the ratio of silicon nitride and the aluminum compounds, the types of elements penetrating to form a solid solution, the proportion of the element becoming the fluorescent center, etc.
It has been found that CaSiAlN3, Ca2(Si,Al)5N8 and a β-sialon activated by a rare earth element also have the similar fluorescence properties (Patent Documents 7 and 8, and Non-Patent Documents 2 and 3).
Other phosphors proposed include phosphors of nitrides and oxynitrides (hereinafter referred to also as nitride phosphors and oxynitride phosphors, respectively) such as aluminum nitride, magnesium silicon nitride, calcium silicon nitride, barium silicon nitride, gallium nitride and zinc silicon nitride.
Further, as sealing resins, epoxy resins have heretofore been used in many cases (Patent Document 1). However, epoxy resins have a drawback that when exposed to ultraviolet rays for a long time, the resins undergo coloration, whereby the light transmittance decreases, and the operating life of the light source will be shortened. To overcome such a drawback, recently, a silicone resin has been used as a sealing resin (Patent Document 9).
However, it is known that a silicone resin is also likely to be deteriorated when exposed to a large amount of ultraviolet rays. Especially, recently, development of high power LED is being pursued, and the power consumption or the light energy density per one chip tends to increase, and accordingly, deterioration of the resin by heat or ultraviolet rays tends to be remarkable.
The white LEDs obtained heretofore are inferior in luminous efficiency to fluorescent lamps, and there is a strong demand for a LED superior in luminous efficiency to the fluorescent lamps. A white LED employing a nitride phosphor or an oxynitride phosphor such as a sialon phosphor has an efficiency higher than an incandescent lamp, but in order to expand its application to general illumination, it is required to enlarge the emission element to increase the output and to attain high luminous efficiency and improvement of the characteristics for illumination.
On the other hand, in the case of an oxynitride phosphor or a nitride phosphor, the excitation spectrum of the phosphor is spread towards the bottom to the vicinity of about 250 nm on the shortest wavelength side, and in order to maximize the absorption of the excitation light, it is desired to employ a resin having high transparency up to the vicinity of 250 nm, but many resins have low light transmittance on the short wavelength side.
White light requires a combination of plural colors as is different from monochromatic light, and a general white LED is composed of a combination of an ultraviolet LED or a blue LED with a phosphor which emits visible light with the LED as an excitation source. Accordingly, in addition to the improvement in efficiency of the white LED, it is necessary to improve the efficiency of the phosphor to be used and further to improve the efficiency in taking out the emitted light to the outside. In order to expand the application of the white LED to general illumination, it is necessary to improve all of such efficiencies.
In many cases, such a phosphor is used as filled in the form of a powder in a resin. At the time of filling the phosphor powder in the resin, especially when the filling rate of the powder is low, sedimentation of the phosphor powder is likely to occur, and consequently, a density distribution is likely to result, thus leading to a problem such that the luminescence properties are unstable. It is widely known to employ a sedimentation-preventing agent of an organic type to prevent such a problem. However, a conventional sedimentation-preventing agent of an organic type is not transparent to the excitation light or emitted light, and thus, when the phosphor powder is filled in a resin at a low rate, there will be a problem such that the luminance tends to deteriorate.
Patent Document 1: JP-A-11-500584
Patent Document 2: JP-A-2002-363554
Patent Document 3: JP-A-2003-336059
Patent Document 4: JP-A-2003-124527
Patent Document 5: JP-A-2003-206481
Patent Document 6: JP-A-2004-186278
Patent Document 7: JP-A-2004-244560
Patent Document 8: JP-A-2005-255895
Patent Document 9: JP-A-2005-136379
Non-Patent Document 1: J. W. H. van Krebel, “On new rare-earth doped M-Si—Al—O—N materials”, T U Eindhoven, The Netherlands, P. 145-161 (1998)
Non-Patent Document 2: Extended Abstracts (The 52nd Spring meeting, March, 2005, Saitama University) p. 1614-1615; The Japan Society of Applied Physics and Related Societies
Non-Patent Document 3: Extended Abstracts (The 65th Autumn Meeting, September, 2004, Tohoku Gakuin University) p. 1282-1284; The Japan Society of Applied Physics